For a method for recording and erasing using a magneto-optical memory device, which utilizes magneto-optical effect, various types of overwriting methods have been proposed, such as the magnetic field modulation and the light intensity modulation. With the magnetic field modulation, a recording operation is carried out by reversing the magnetization direction while successively projecting a laser beam. Whereas, the recording operation is carried out by modulating the intensity of the laser beam with the light intensity modulation. However, with the magnetic field modulation, there is a limit in improving the recording speed because of the time required for reversing the magnetic head which reverses the magnetization direction. Therefore, for high speed memory, the magneto-optical memory device employing the light intensity modulation has been researched and developed.
For the magneto-optical memory device which employs the light intensity modulation, a magneto-optical memory device consisting of an initialization-use magnet and a double-layered film has been proposed. (The 53rd meeting on Magnetics by Japan Magnetic Society (1988), p.78)
With the memory device of double-layer structure, an overwriting operation is carried out through the light intensity modulation by the following way.
A recording film, whereon information is recorded, consists of a recording layer and a writing layer laminated in a direction perpendicular to the surface of the recording film. The recording layer has its Curie temperature that is below the Curie temperature of the writing layer, and high coercive force at room temperature. Furthermore, the magnetic field to be applied from the initialization-use magnet is greater than the coercive force of the writing layer but smaller than the coercive force of the recording layer. First, the magnetization direction of the writing layer is arranged in the magnetization direction of the initialization-use magnet.
Next, a light beam with high level or low level intensity is projected. When projecting the light beam with low level intensity, the temperature of the writing layer is not raised to the Curie temperature. The magnetization direction of the writing layer is therefore maintained and copied to the recording layer by the exchange coupling force. On the other hand, in the case of high level intensity, both recording layer and the writing layer are raised above the respective Curie temperatures. Thus, the writing layer is magnetized in the magnetization direction of an externally-applied recording magnetic field. Thereafter, the magnetization direction of the recording layer is arranged in the same direction.
However, the discussed arrangement creates the following problems: Since the magnetic field of the initialization use magnet, whose intensity is ten times as strong as that of the magnetic field for recording, is required for the initialization-use magnet, the initialization-use magnet becomes larger in size, and therefore the recording-reproduction apparatus becomes larger in size and more complicated. In order to obtain the recording film wherein exchange coupling force is exerted between the two layers, the selection of the composition and the manufacturing process for the recording film are complicated. Furthermore, the material whose Curie temperature is below the Curie temperature of the writing layer must be selected for the recording layer. For this reason, the Kerr rotation angle of the recording layer becomes small, and the high quality reproduction signal may not be obtained.